(CBS/AP) WASHINGTON - President Barack Obama, responding to the explosions at the Boston Marathon, says the United States does not know "who did this or why" but vowed that whoever is responsible "will feel the full weight of justice."

Speaking from the White House just three hours after the explosions, Obama was cautious in his remarks, stopping short of calling the incident an act of terror. But he vowed to hold those responsible accountable.

"We will find out who did this. We'll find out why they did this," Obama said in his brief, three-minute statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."

"...Boston is a tough and resilient town - so are its people," he continued. "I am supremely confident that Bostonians will pull together, take care of each other and move forward as one proud city. And as they do, the American people will be with them every single step of the way."

Obama made his remarks Monday evening from the White House about three hours after two explosions detonated near the marathon's finish line. At least two people were killed and 50 injured in the blasts.

Obama did not offer specific details on deaths or injuries, saying only that multiple people had been wounded, some of them gravely.

After being notified about the situation around 3 p.m. ET, the president was briefed in the Oval Office by Lisa Monaco, deputy national security adviser for Homeland Security and Counterterrorism; FBI Director Robert Mueller and Homeland Security Secretary Janet Napolitano followed up with reports on the active investigation and response, including ongoing coordination with state and local officials.

Mr. Obama then placed calls to Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick, offering his condolences as well as the full support of the federal government in the immediate, medium and long-term response to the tragedy.

Attorney General Eric Holder, after speaking with Mueller and U.S. Attorney Carmen Ortiz, also directed the full resources of the Justice Department to be deployed to ensure the matter is fully investigated.

U.S. Secret Service spokesman Ed Donovan said in a statement that "out of an abundance of caution, we have expanded our security perimeter at the White House complex" - a move he added is "not unusual."

The president said the government would increase security around the United States "as necessary" but did not say whether his administration thought the incident was part of a larger plot.